The present invention relates to commercial vehicles; and more particularly to drivetrain control strategies for such vehicles as heavy trucks, buses and the like.
In the course of driving heavy vehicles such as overland trucks and buses (which should be considered interchangeable for purposes of the description contained herein), it is common to be required to drive at relatively slow speeds, often for extended periods of time. Exemplary situations are driving in slow, backed up traffic and maneuvering about loading yards where high-speed travel is not possible. In modern heavy vehicles, it is common to find that such vehicles are equipped with a semi-automatic transmission or an automatic mechanical transmission (AMT) or a power-shifting automatic transmission. In any of the above cases, computer control strategies are utilized in the selection of gear engagements, as well as transition strategies between the different gear choices of the transmission. Furthermore, in a stepped transmission some gearwheels and shafts that are used for torque transfer in different gears are rotationally fixed to each other by means of connecting devices, for instance clutches, hi a positive connecting device, such as a dog clutch, the torque is transferred substantially by normal forces, as opposed to a frictional connecting device, such as a plate clutch, where the torque is transferred substantially by friction forces. The difference between the different types of available stepped transmissions can be described as the operation of clutch, gear selection, and carrying out the gear selection, hi an automatic mechanical transmission the operation of the clutch, gear selection and the carrying out of the gear selection is performed automatically without driver intervention. A manual transmission requires the driver to perform the operation of the clutch, gear selection and the carrying out of the gear selection. Manual transmissions are typically of the mechanically engaged type in general. A semi-automatic transmission is one in which one or more of the operation of the clutch, gear selection, or carrying out gear selection is performed by the driver of the truck. The semi-automatic transmission may also be of the mechanical type transmission. In a transmission of the mechanically engaged type, there are positive connecting devices. During a gear shift, there will normally be an interruption of the torque transfer in a transmission of the mechanically engaged type. Frictional connecting devices are mainly found in power-shifting transmission, where torque is also transferred during a gear shift. Power-shifting transmissions are usually automatic or semi-automatic. Automatic transmissions of the mechanically engaged type are referred to as automatic mechanical transmissions. Such a transmission may use a manual transmission with controllers to automate the operation of the transmission. It may also be specifically designed to be automatic and not based on a manual transmission. Hereinafter, the automatic mechanical transmission and power-shifting automatic transmission are referred to as an automatic transmission.
Referring to the situations in which it is desired that the heavy vehicle moves slowly but substantially constantly on course, operators have developed habits for engaging an appropriate low gear which carries the vehicle forward or backward under the power of the idling engine. Thus, the driver of the vehicle is idle driving, i.e. without pressing an accelerator pedal, or any other drive torque controlling device, arranged in the vehicle. Depending upon the desired speed and the heavy vehicle load, among other factors, different low gears are selectable.
The low gears available for selection, however, are limited by the torque that can be developed in each gear by the engine operating at the preset idle speed, and the range of gears available for use at any particular time will be determined by conditions of the vehicle, as well as conditions of the environment within which the vehicle is operating. The two primary conditions upon which the range of available gears is dependent is the mass of the vehicle (including any load) and ground inclination. Dependent at least in part on each of these two characteristics, the highest gear of the transmission can be determined at which the idling engine can maintain a substantially constant speed of the vehicle without losing speed because of insufficient torque capability. Heretofore, drivers have been left to draw on their experience for selecting an initial gear for such idle travel, with adjustments being made up or down in order to engage the gear which produces the desired travel speed, and which is also capable of maintaining that speed using the torque developed at the preset idle speed of the engine, for example, 650 revolutions per minute.
It is appreciated that if presently existing conditions are known which bear upon the highest gear selection at which the idling engine can maintain a constant vehicle speed, that gear can be determined, engaged and utilized for powering travel of the vehicle. Often times, however, the gear ratio carries the vehicle at a groundspeed slower than desired. For instance, the traffic speed within which the heavy truck is operating maybe faster than this speed which the idling engine can maintain under existing conditions. Heretofore, as described above, selection of the proper gear which permits the engine to operate at idle and produce the desired higher speed of the vehicle was performed by the driver himself based on past experience and trial-and-error with respect to selection within a typical low range of gears and the selection of accelerator pedal position to achieve the right vehicle acceleration and finally the right vehicle speed.
This type of trial-and-error, hunt-and-peck of gear selection and accelerator pedal position by the driver obviously has drawbacks; among others, if the truck is operating under slow speed conditions, the driver can become unnecessarily fatigued by the selection process. Still further, operating economy can suffer not only because of inefficiencies associated with constant gear changing and adjustments of the accelerator pedal position, but also if the optimal gear is not selected which can use the preset idle speed of the engine for maintaining the desired vehicle speed. Therefore, the need has been recognized for a drivetrain control system in which such gear selections and selection of accelerator pedal position are made on at least a semi-automated basis with only minimal or no direct selection input from the driver.
In at least one embodiment, the present invention takes the form of a method for operating a semi-automatic or automatic transmission of a heavy vehicle when driving at idle speed. The method according to the invention comprises (includes, but is not necessarily limited to) supplying fuel to the engine of the heavy truck at a rate that facilitates engine-idle operation. In another step, the method engages the semi-automatic or automatic transmission in a gear higher than the starting gear of the transmission and permits the vehicle to operate at a first substantially uniform driving velocity under engine-idle power. Depending upon traffic and environmental requirements which require a faster speed, the driver upshifts the semi-automatic or automatic transmission by manually controlling a control device for manual gear selection and then drives the vehicle at a second substantially uniform driving velocity under engine-idle power. Necessarily, the second substantially uniform driving velocity is higher than the first substantially uniform driving velocity. The method according to the invention gives an increased driving comfort and fuel savings. The method also decreases unnecessary clutch slip.
In another embodiment of the invention by controlling the engine speed of the vehicle the engine speed is automatically increased up to a predetermined vehicle speed before the upshift is performed. In still another embodiment the speed of the vehicle is automatically increased after the upshift by reengaging the clutch. Thus synchronization of engine idling speed to the new gear is performed by the clutch, m another embodiment the speed of the vehicle is increased by performing a combination of engine control and clutch control, hi still another embodiment the vehicle speed increase is dependent of and adapted to current vehicle travel resistance. The upshift function according to the invention could also only be available when travel resistance of said vehicle is below a predetermined value. This could also include predicted future travel resistance. In another embodiment of the invention the manual driver control of the control device causes the transmission to upshift one gear, and in still another embodiment a longer manual control (such as e.g. depression) of the control device can cause the transmission to upshift at least two gears. In still another embodiment of the invention a certain number of manual control actions of the control device cause the transmission to upshift corresponding number of gear steps.
Further advantageous embodiments of the invention emerge from the following description.